Conformational Rearrangement within STIM1 C-terminus Crucial for Coupling to Orai1

Ca2+ influx in non-excitable cells is mainly carried by store-operated channels (SOCs), where Orai1 (CRACM1) and STIM1 represent the two molecular key players in this process. STIM1 functions as an endoplasmic reticulum located Ca2+ sensor and transmits the signal of store depletion to the plasma membrane by coupling to Orai1, which in turn causes channel activation. STIM1 C-terminus itself can act as a surrogate of full length STIM1 and is sufficient for the activation of Orai1 currents. 2-aminoethoxydiphenylborate (2-APB) has been shown to induce enhanced association of STIM1 C-terminus with Orai1 suggesting a conformational change within the former that drives this association. Indeed we were able to monitor 2-APB induced STIM1 C-terminal conformational rearrangement by fluorescence microscopy. In the absence of 2-APB the change in conformation was only seen for STIM1 C-terminus coupled to Orai1 but not for that part remaining in the cytosol suggesting this conformational change crucial for Orai1 binding. Moreover, we were able to mimic this conformational rearrangement by introducing selective point mutations into STIM1 C-terminus, which in line substantially increased binding to Orai1. In aggregate, our data support the theory of flexible regions within STIM C-terminus that undergo conformational rearrangement upon coupling to Orai1. (Supported by FWF-P21118)